1. Field of the Invention
The present invention relates to a diffracting device having a distributed bragg reflector, a wavelength changing device having an optical waveguide with periodically inverted-polarization layers, a laser beam generating apparatus in which a laser beam having a fixed wavelength is generated with the diffracting device or the wavelength changing device, an optical information processing apparatus in which information is optically read or written with the laser beam generating apparatus, and an integrated optical circuit in which a laser beam having a fixed waveguide is converged with integrated devices, in an optical information processing field, an optical applied measuring control field and an optical communication field in which coherent light is utilized. Also, the present invention relates to a manufacturing method of the diffracting device and a manufacturing method of the wavelength changing device.
2. Description of the Related Art
A diffracting device is important to be utilized for not only a device having an optical waveguide but also a light integrated circuit. In cases where a plurality of gratings are periodically arranged in an optical waveguide to manufacture a diffracting device, light propagated through the optical waveguide is controlled by the gratings. For example, in cases where the gratings periodically arranged in the optical waveguide act as a distributed Bragg reflector, coherent light having a particular wavelength is selectively reflected in the optical waveguide, and the coherent light reflected is propagated through the optical waveguide in the opposite direction.
2.1. FIRST PREVIOUSLY PROPOSED ART:
A conventional diffracting device is described with reference to FIG. 1.
FIG. 1 is a diagonal view of a conventional diffracting device having a distributed Bragg reflector.
As shown in FIG. 1, a conventional diffracting device 11 consists of a LiNbO.sub.3 substrate 12, a plurality of gratings 13 periodically arranged in series in a central surface of the substrate 12 at regular intervals .LAMBDA.1, and a Ti diffused optical waveguide 14 extending from one side of the substrate 12 to the other side through the gratings 13. In the above configuration, light beams having various wavelengths are radiated to an incident surface 14a positioned at one side of the optical waveguide 14, and a particular light beam having a particular wavelength is reflected by the gratings 13 because periodic change in a refractive index of the optical waveguide 14 is formed by the gratings 14 periodically arranged. That is, the gratings 13 act as a distributed Bragg reflector. Therefore, the particular light beam is output from the incident surface 14a of the optical waveguide 14, and remaining light beams except the particular light beam are output from an output surface 14b positioned at the other side of the optical waveguide 14.
Next, a conventional manufacturing method of the conventional diffracting device 11 is described with reference to FIGS. 2A to 2D. The method is performed with two superimposed masks (J. SOCHTIG, "Ti:LiNbO.sub.3 Stripe Waveguide Bragg Reflector Grating", Electronics Letters, Vol. 24, No. 14, p. 844-845 (1988)).
As shown in FIG. 2A, after the optical waveguide 14 is formed by diffusing Ti into a central surface region of the substrate 12, a thin Ti film 15 is deposited on the substrate 12 and the optical waveguide 14. The thin Ti film 15 is utilized as a first superimposed mask. Thereafter, a photoresist 16 is spin coated on the Ti film 15. The photoresist 16 is utilized as a second superimposed mask. Thereafter, as shown in FIG. 2B, the photoresist 16 is exposed to interference light according to an interference-exposure process, and the photoresist 16 exposed is developed to remove exposed areas of the photoresist 16. Therefore, a periodic grating pattern is transferred to the photoresist 16. Thereafter, as shown in FIG. 2C, the Ti film 15 is periodically etched at regular intervals .LAMBDA.1 by reactive ions generated in an atmosphere of CCl.sub.2 F.sub.2 gas according to a reactive ion etching to transfer the periodic grating pattern of the photoresist 16 to the Ti film 15. Thereafter, as shown in FIG. 2D, the patterned film 15 is used as a mask, and the LiNbO.sub.3 substrate 12 is etched at the regular intervals .LAMBDA.1 by reactive ions generated in an atmosphere of CF.sub.4, Ar, and N.sub.2 according to the reactive ion etching. Therefore, the gratings 13 are periodically formed in surface portions of the substrate 12 at the regular intervals .LAMBDA.1. Thereafter, both sides of the optical waveguide 14 are polished.
FIG. 3 graphically shows transmitting and reflecting characteristics of the diffracting device 11.
As shown in FIG. 3, when light beams having wavelengths of 1.5 .mu.m band are radiated from a light emitting diode and are coupled to the optical waveguide 14 of the diffracting device 11, a particular light beam having a particular wavelength .lambda.p which satisfies a Bragg condition is selectively reflected. The Bragg condition is determined by regular intervals of the gratings 13 and the effective refractive index of the grating.
2.2. SECOND PREVIOUSLY PROPOSED ART:
FIG. 4 is a cross-sectional view of another conventional diffracting device.
As shown in FIG. 4, another conventional diffracting device 21 consists of a glass substrate 22, an optical waveguide 23 formed in a central surface portion of the substrate 22 according to an ion-exchange process, and a plurality of SiO.sub.2 gratings 24 periodically arranged at regular intervals .LAMBDA.1=1.2 .mu.m. A total length of the SiO.sub.2 gratings 24 is 10 mm in a propagation direction of the coherent light.
A distributed Bragg reflector is formed by a periodic structure composed of the SiO.sub.2 gratings 24 and spaces between the gratings 24 in cases where a distributed Bragg reflector condition (or DBR condition) .LAMBDA.1=m.lambda./2N is satisfied. Here the symbol .LAMBDA.1 denotes the regular intervals of the gratings 24, the symbol m is a grating order off the periodic structure, the symbol .lambda. denotes a wavelength of coherent light, and the symbol N denotes an averaged refractive index of the periodic structure. When the wavelength of the coherent light is 1.3 .mu.m, the DBR condition is satisfied to reflect the coherent light in the periodic structure of which the grating order m is equal to 3.
In the above configuration, coherent light converged at an incident end facet 23a transmits through the optical waveguide 23. In this case, a part of the coherent light is distributed off the optical waveguide 23, so that the coherent light distributed off the optical waveguide 23 is reflected by the gratings 24.
Next, a manufacturing method of the diffracting device 21 is described.
After the optical waveguide 23 is formed in the substrate 22, a SiO.sub.2 film is deposited on the optical waveguide 23 and the substrate 22. Thereafter, a photoresist film is coated on the SiO.sub.2 film. Thereafter, grating pattern areas of the photoresist is selectively exposed to ultraviolet radiation according to a conventional interference-exposure process, and the photoresist is developed to remove the grating pattern areas of the photoresist. Therefore, a grating pattern is transferred to the photoresist film. Thereafter, the SiO.sub.2 film etched by reactive ions according to a dry etching while the photoresist film is utilized as a mask. Therefore, the grating pattern is transferred to the SiO.sub.2 film, and the gratings 24 made of SiO.sub.2 are formed on the optical waveguide 23.
When 1.3 .mu.m wavelength coherent light is coupled to the optical waveguide 23, 5% of the coherent light is reflected by the SiO.sub.2 gratings 24.
2.3. PROBLEMS TO BE SOLVED BY THE INVENTION:
However, because the substrate 12 is made of a hard material LiNbO.sub.3, complicated processes are required to directly etch the substrate 12 in the conventional diffracting device 11. Also, it is difficult to etch the gratings 13 made of the hard material by a predetermined depth. Therefore, the reprobability of the apparatus 11 deteriorates, and the gratings 13 are often excessively etched. Also, the surfaces of the gratings 13 become rough because of the radiation of the reactive ions. Therefore, light beams transmitting through the optical waveguide 14 are increasingly scattered. In the same manner, because the gratings 24 on the substrate 22 are made of a hard material SiO.sub.2, complicated processes are required to form the gratings 24 according to an etching process in the conventional diffracting device 21. Also, it is difficult to etch the gratings 24 without erroneously etching the optical waveguide 23 according to a dry etching process. Therefore, the reprobability of the apparatus 21 deteriorates, and the gratings 24 are often excessively etched to etch the optical waveguide 23. As a result, the surfaces of the optical waveguide 24 become rough so that the coherent light is increasingly scattered.
Also, it is difficult to etch material having a large refractive index and a large transmission coefficient because an etching rate of those materials is very low in general. Therefore, it is troublesome to deeply form the gratings 13, 24. As a result, it is difficult to reflect the light with high reflecting efficiency. Also, because the gratings 13, 24 are formed according to the complicated processes in which a plurality of pattern transferring processes are performed, the unevenness of the periodic pattern in the gratings 13, 24 is increased. Therefore, as shown in FIG. 3, though the reflection of the light beams theoretically occurs at a single particular wavelength .lambda.p, the reflection of the light beams actually occurs in a wide wavelength range. In other words, the condition that the gratings 13, 24 function as the distributed Bragg reflector deteriorates because of the complicated processes.
Also, because the unevenness of the periodic pattern in the gratings 13, 24 is increased and because the light transmitting through the optical waveguides 14, 23 is scattered by the roughness of the gratings 13 and the optical waveguide 23, a transmission loss of the fundamental waves is increased. Therefore, the intensity of the light is lowered, and a diffraction efficiency of the gratings 13, 24 is lowered.
Also, because the position of the gratings 13, 24 is limited near to the surfaces of the substrates 12, 22, the intensity of the light reflected by the gratings 13, 24 is limited. Therefore, it is difficult to reflect the light with high reflecting efficiency unless the length of the gratings 13, 24 extending in a propagation direction is extremely lengthened to increase the number of gratings 13, 24.
2.4. THIRD PREVIOUSLY PROPOSED ART:
A wavelength changing device having an optical waveguide has been proposed. The optical waveguide is provided with alternate rows of non-inverted and inverted polarization layers to change fundamental waves transmitting through the optical waveguide to second harmonic waves. The inverted polarization layers are formed by compulsorily inverting the non-linear polarization of ferroelectric substance. The wavelength changing device is utilized for a small-sized shorter wavelength laser beam generating apparatus because fundamental waves radiated from a semiconductor laser are changed to second harmonic waves such as a green or blue light. Therefore, the wavelength changing device is useful in a printing operation, an optical information processing, an optical applied measuring control field, and an optical communication field.
The wavelength change in the wavelength changing device can be performed with high efficiency because fundamental waves radiated from a semiconductor laser are changed to second harmonic waves in the alternate rows of non-inverted and inverted polarization layers. Also, because the wavelength of the fundamental waves changed to the second harmonic waves depends on regular intervals of the alternate rows, the wavelength of the second harmonic waves obtained in the wavelength changing device can be arbitrarily changed. However, because the regular intervals of the alternate rows in the wavelength changing device are fixed, the output power of the second harmonic waves considerably fluctuates when the wavelength of the fundamental waves radiated from a semiconductor laser fluctuates.
For example, the change of wavelength in a shorter wavelength laser beam generating apparatus has been proposed (K. Yamamoto et. al, "Milliwatt-Order Blue-light Generation in a Periodically domain-Inverted LiTaO.sub.3 waveguide", Optica letters, Vol. 16, No. 15, p. 1156-1158, (1991)). In the laser beam generating apparatus of Yamamoto, fundamental waves of semiconductor laser beams are changed to second harmonic waves in an optical waveguide having alternate rows of non-inverted and inverted polarization layers according to quasi-phase matching.
FIG. 5 is a constitutional view of a conventional shorter wavelength laser beam generating apparatus.
As shown in FIG. 5, a conventional shorter wavelength laser beam generating apparatus 31 consists of a semiconductor laser 32, a collimator lens 33 for collimating fundamental waves radiated from the semiconductor laser 32, a .lambda./2 plate 34 for rotatively polarizing the fundamental waves, a focusing lens 35 having a numerical aperture NA=0.6, and a wavelength converting device 36 having an optical waveguide 37 for changing the fundamental waves converged at an incident end facet 37a to second harmonic waves such as blue light according to the quasi-phase matching. The optical waveguide 37 is provided with alternate rows of non-inverted and inverted polarization layers. The incident end facet 37a and an output end facet 37b of the optical waveguide 37 are coated with antireflection coating to prevent the fundamental waves from being reflected in the incident and output end facets 37a, 37b.
In the above configuration, 874 nm wavelength fundamental waves are radiated from the semiconductor laser 32 and are collimated by the collimator lens 34. Thereafter, the fundamental waves are rotatively polarized by the .lambda./2 plate 34 and are converged at the incident end facet 37a of the optical waveguide 37 by the focusing lens 35. In this case, though the antireflection coating is coated on the incident end facet 37a, approximately 1% of the fundamental waves are fed back to the semiconductor laser 32 in practical use. Thereafter, blue light consisting of 437 nm wavelength second harmonic waves are radiated from the output end facet 37b of the optical waveguide 37 on condition that a quasi-phase matching condition formulated by an equation .LAMBDA.2=.lambda..sub.f /{2*(N2.omega.-N.omega.)} is satisfied. Here the symbol .LAMBDA.2 denotes regular intervals of the alternate rows in the optical waveguide 37, the symbol .lambda..sub.f denotes a wavelength of the fundamental waves, the symbol N2.omega. denotes an effective refractive index of the non-inverted and inverted polarization layers for the second harmonic waves, and the symbol N.omega. denotes an effective refractive index of the non-inverted and inverted polarization layers for the fundamental waves.
Accordingly, the fundamental waves such infrared light can be reliably changed to blue light. For example, when the pumping power of the fundamental waves converged at the incident end facet 37a of the optical waveguide 37 is 35 mW, the pumping power of the blue light radiated from the output end facet 37b is 1.1 mW.
However, because the blue light is generated by changing the fundamental waves to the second harmonic waves and multiplying the second harmonic waves in the optical waveguide 37 in which the alternated rows of the non-inverted and inverted polarization layers are arranged at regular intervals, a wavelength range of the fundamental waves allowed to obtain the second harmonic waves is only 0.2 nm in the optical waveguide 37. Also, the wavelength of the fundamental waves radiated from the semiconductor laser 32 fluctuates depending on the ambient temperature of the semiconductor laser 32. The fluctuation ratio of the wavelength to the ambient temperature is about 0.2 nm/.degree. C. Therefore, in cases where the ambient temperature of the semiconductor laser 32 varies by 1.degree. C., the blue light cannot be generated in the optical waveguide 37.
In addition to the fluctuation of the ambient temperature, the amplification mode of the fundamental waves radiated from the semiconductor laser 32 varies because approximately 1% of the fundamental waves converged at the incident end facet 37a of the optical waveguide 37 is fed back to the semiconductor laser 32. In this case, the wavelength of the fundamental waves radiated from the semiconductor laser 32 varies about 1 nm after a short time. Therefore, the stable change period of the fundamental waves to the second harmonic waves is no more than several seconds.
Accordingly, the stabilization of the wavelength of the fundamental waves is required to stably generate the blue light in the conventional shorter wavelength laser beam generating apparatus 31.
2.5. FOURTH PREVIOUSLY PROPOSED ART:
To stably change fundamental waves to second harmonic waves with a wavelength changing device according to the quasi-phase matching, a wavelength changing device having a plurality of gratings periodically arranged has been proposed (K. Shinozaki, et. al, "Self-Quasi-Phase-Matched Second-Harmonic Generation in the Proton-Exchanged LiNbO.sub.3 Optical Waveguide with Periodlcally Domain-Inverted Regions", Apply. Phys. Lett., Vol. 59, No. 29, p. 510-512(1991)).
FIG. 6 is a constitutional view of another conventional shorter wavelength laser beam generating apparatus in which a conventional wavelength changing device of Shinozaki is arranged.
As shown in FIG. 6, a conventional shorter wavelength laser beam generating apparatus 41 consists of a semiconductor laser 42, a conventional wavelength changing device 43 for changing 1.3 .mu.m wavelength fundamental waves radiated from the semiconductor laser 42 to 0.65 .mu.m wavelength second harmonic waves, a spectrum analyzer 44 for analyzing the wavelength of the fundamental waves radiated from the semiconductor laser 42, and two pairs of optical lenses 45 for converging the fundamental waves radiated from the semiconductor laser 42 at single mode fibers connected to the wavelength changing device 43 and the spectrum analyzer 44. The wavelength changing device 43 consists of a polarized LiNbO.sub.3 substrate 46, an optical waveguide 47 having inverted polarization layers 48 (or domain-inverted regions) periodically arranged at regular intervals .LAMBDA.. Regions between the inverted polarization layers 48 are called non-inverted polarization layers 49 for convenience.
In the optical waveguide 47, mismatching between a propagation constant of the fundamental waves and another propagation constant of the second harmonic waves is compensated by alternate rows of the inverted and non-inverted polarization layers 48, 49. This is, because the difference in the propagation constant between the fundamental waves and the second harmonic waves occurs, the phase of the fundamental waves agrees with that of the second harmonic waves in the optical waveguide 47 each time the fundamental waves transmit a minimum distance. Therefore, in cases where the regular intervals .LAMBDA. of the inverted polarization layers 48 agree with a multiple of the minimum distance, the quasi-phase matching condition .LAMBDA.=.lambda..sub.f /{2*(N2.omega.-N.omega.)} is satisfied, and the fundamental waves are changed to the second harmonic waves. The condition that the regular intervals .LAMBDA. of the inverted polarization layers 48 agree with the minimum distance is called a first-order quasi-phase matching. Also, the condition that the regular intervals .LAMBDA. agree with N times minimum distance is called an Nth-order quasi-phase matching.
In the above configuration, fundamental waves having various wavelengths around 1.3 .mu.m are radiated from the semiconductor laser 42 and are converged at the optical waveguide 47 through the optical lenses 45 and the single mode fiber. In the optical waveguide 47, quasi-phase matching (QPM) fundamental waves having a QPM wavelength satisfying the quasi-phase matching condition are selectively changed to second harmonic waves, and the second harmonic waves are efficiently amplified and output from the optical waveguide 47. Therefore, the QPM fundamental waves are selectively changed to the second harmonic waves in the wavelength changing device 43.
In addition, because an effective refractive index of the inverted polarization layers 48 is slightly higher than another effective refractive index of the non-inverted polarization layers 49, a periodic structure in the effective refractive index consisting of the inverted polarization layers 48 and the non-inverted polarization layers 49 is produced in the optical waveguide 47. Therefore, a plurality of gratings are substantially formed in the optical waveguide 47. A group of the gratings substantially formed functions as a distributed Bragg reflector on condition that the DBR condition .LAMBDA.=m.lambda./2N is satisfied. That is, DBR fundamental waves having a DBR wavelength satisfying the DBR condition are selectively reflected in the gratings. Thereafter, the reflected DBR fundamental waves are fed back to the semiconductor laser 42. Therefore, the wavelength of the fundamental waves radiated from the semiconductor laser 42 is fixed to the DBR wavelength.
Accordingly, in cases where the DBR wavelength of the DBR fundamental waves reflected in the periodic structure functioning as the distributed Bragg reflector agrees with the QPM wavelength of the QPM fundamental waves, the change of the fundamental waves to the second harmonic waves can be stably performed in the conventional shorter wavelength laser beam generating apparatus 41.
To achieve an agreement of the DBR wavelength of the reflected DBR fundamental waves and the QPM wavelength of the QPM fundamental waves, regular intervals a of the inverted polarization layers 48 periodically arranged are set to 13 .mu.m .mu.m. In this case, the wavelength of the fundamental waves radiated from the semiconductor laser 42 is fixed to 1.327 .mu.m, and 1.327/2 .mu.m wavelength second harmonic waves are stably generated. Also, the alternate rows of the inverted and non-inverted polarization layers 48, 49 becomes a first-order in the QPM structure, and the gratings functioning as the distributed Bragg reflector becomes a forty-third order in the DBR periodic structure. The grating order m is defined as an equation m=.LAMBDA./(.lambda..sub.f /2N). Here the symbol A denotes the regular intervals of the inverted polarization layers 48, the symbol .lambda..sub.f denotes a wavelength of the fundamental waves, and the symbol N denotes an effective averaged refractive index of the optical waveguide 47 for the fundamental waves. In cases where the pumping power of the fundamental waves converged at the optical waveguide 47 is 60 .mu.W and the length of the optical waveguide 47 is 2 mm, the output power of the second harmonic waves is 0.652 pW.
2.6. PROBLEMS TO BE SOLVED BY THE INVENTION:
However, because the inverted polarization layers 48 periodically arranged function as a distributed Bragg reflector grating in the conventional shorter wavelength laser beam generating apparatus 41, a propagation speed of the fundamental waves and another propagation speed of the second harmonic waves are required to be controlled with high accuracy to achieve the agreement of the DBR wavelength of the DBR fundamental waves and the QPM wavelength of the QPM fundamental waves.
Also, the range of the wavelength of the fundamental waves changed in the apparatus 41 is limited. Therefore, even though 1.3 .mu.m wavelength fundamental waves can be stably changed to 0.65 .mu.m wavelength second harmonic waves, there is a drawback that shorter wavelength second harmonic waves (the wavelengths range from 400 nm to 500 nm) useful in various fields are difficult to be generated in the apparatus 41.
Also, because the inverted polarization layers 48 periodically arranged are utilized as the distributed Bragg reflector in the conventional shorter wavelength laser beam generating apparatus 41, the grating order in the DBR periodic structure becomes large in the apparatus 41. For example, in cases where the alternate rows of the inverted and non-inverted polarization layers 48, 49 is equivalent to the first-order in the QPM structure, the periodic structure functioning as the distributed Bragg reflector is equivalent to a several tens of grating order in the DBR periodic structure. Therefore, the fundamental waves are coupled to various radiation modes in the optical waveguide 47. The radiation modes consists of N types of radiation modes from a first radiation mode corresponding to the first grating order to an Nth radiation mode corresponding to an Nth grating order in cases where the periodic structure of the inverted polarization layers 48 is equivalent to the Nth grating order. Thereafter, the fundamental waves are radiated to various directions without being changed to the second harmonic waves while being led by the various radiation modes. As a result, the fundamental waves attenuates in the optical waveguide 47, and a radiating loss of the fundamental waves is increased. Accordingly, because the fundamental waves contributing the generation of the second harmonic waves are decreased by the increase of the radiating loss, there is a drawback that a changing efficiency of the fundamental waves to the second harmonic waves deteriorates. This drawback is illustrated in FIG. 7.
FIG. 7 graphically shows a relationship between a reflection efficiency of the fundamental waves and the grating order and another relationship between a radiation loss of the fundamental waves and the grating order. As shown in FIG. 7, in cases where the gratings are arranged in a tenth grating order periodic structure, the reflection efficiency is only 10%, and the radiation loss is no less than 75%. Therefore, in cases where the grating order of periodic structure in the the distributed Bragg reflector grating is equal to or more than third grating order, the radiation loss of the fundamental waves is too many so that the conventional shorter wavelength laser beam generating apparatus 41 is not useful in practical use.
In addition, higher grating order of the DBR periodic structure adversely influences on not only the fundamental waves but also the second harmonic waves generated in the optical waveguide 47 to increase a radiation loss of the second harmonic waves. Therefore, the second harmonic waves are scattered and reflected in the optical waveguide 47 to decrease the second harmonic waves radiated from an output end facet 47b of the optical waveguide 47. As a result, there is a drawback that the changing efficiency of the fundamental waves to the second harmonic waves moreover deteriorates. Accordingly, a wavelength changing device having the DBR periodic structure of a lower grating order (a first grating order or a second grating order) is required to change the fundamental waves to the second harmonic waves at high efficiency in practical use.